T4 Test
Also known as: Total T4, Thyroxine, Total Thyroxine
What is T4?
Total T4 measures all thyroxine in the blood, both bound to proteins and free. It reflects overall thyroid hormone production.[MedlinePlus (NIH)]
What This Test Measures
This test measures the total amount of T4 hormone in your blood. Most T4 is bound to proteins; only a small fraction is free.[Mayo Clinic]
Why It's Important
Total T4 can be affected by protein levels and is less reliable than Free T4 in some situations.[Cleveland Clinic]
It is still useful for screening and monitoring thyroid disorders.[MedlinePlus (NIH)]
Who Should Get This Test?
Total T4 is less commonly ordered than Free T4 but useful in specific situations.
Risk Factors
- Newborn screening for congenital hypothyroidism
- When Free T4 assay may be unreliable
- Pregnancy thyroid monitoring (with T3 resin uptake for Free Thyroxine Index)
- Suspected protein binding abnormalities
- Monitoring certain thyroid conditions
- When Free T4 assay is not available
Screening Schedule
Routine newborn screening. Otherwise ordered based on clinical indication.
What Happens During the Test
A blood sample is drawn from a vein. For newborns, a heel prick blood sample is used for screening.
Duration
5 minutes for the blood draw; results usually available within 1-2 days
Discomfort Level
Minimal - brief pinch from needle insertion
Risks & Side Effects
Total T4 testing through blood draw carries very few risks.
- •Minor pain or bruising at the needle site
- •Slight bleeding
- •Rarely, lightheadedness
- •Very rare risk of infection
Normal Range
5.0 to 12.0 mcg/dL.
Note: Pregnancy and estrogen increase binding proteins, raising total T4.
Source: MedlinePlus (NIH)Unit Conversion
Convert between mcg/dL and nmol/L
| mcg/dL | nmol/L |
|---|---|
| 4 | 51.5 |
| 6 | 77.2 |
| 8 | 103 |
| 10 | 128.7 |
| 12 | 154.4 |
| 14 | 180.2 |
High Values
High Total T4 can indicate hyperthyroidism or elevated binding proteins (pregnancy, estrogen therapy).[Mayo Clinic]
Low Values
Low Total T4 can indicate hypothyroidism or low binding proteins (malnutrition, liver disease).[Cleveland Clinic]
Test Limitations & Accuracy
Total T4 is affected by protein binding and may not reflect true thyroid status.
- ⚠Pregnancy increases binding proteins, raising Total T4 without hyperthyroidism
- ⚠Estrogen therapy elevates Total T4
- ⚠Liver disease and nephrotic syndrome lower binding proteins
- ⚠Severe illness affects protein levels
- ⚠Free T4 is generally more reliable for assessing thyroid function
- ⚠Genetic variations in binding proteins affect results
- ⚠Certain medications alter binding protein levels
Alternative & Complementary Tests
Free T4 is generally preferred over Total T4 for most clinical situations.
Preferred test - not affected by protein binding
Primary screening test for thyroid function
Calculated from Total T4 and T3 resin uptake
Used with Total T4 to calculate Free Thyroxine Index
References
Related Condition Guides
Medical Disclaimer
This information is for educational purposes only and is not intended as medical advice. Always consult with a qualified healthcare provider for interpretation of test results and medical decisions.
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